Manufactured on the Mechanicus' Forge Worlds, these embodiments of the Omnissiah stalk the battlefields of the Imperium, utilising arcane technology and terrible weaponry to unleash their apocalyptic fury upon the Emperor of Mankind's foes. These great war machines are revered as much for their long service and past deeds as they are for the superior firepower they can unleash upon the enemies of Mankind.

During the Age of Terra the human race advanced beyond its ancient pre-industrial past to obtain spacefaring capability. In this ancient time, Mankind slowly and painstakingly began to settle the habitable worlds in its own solar system and in the star systems near its homeworld using massive starships capable of only sublight speeds.

Mars was one of the first colony worlds to be settled by humanity, if not the first. But during the Age of Strife, interstellar travel and communications became erratic as Warp Storms became more frequent and more intense as the birth pangs of the Chaos GodSlaanesh roiled the Immaterium. The use of Warp-Drives and Astropaths to tie together human-settled space became increasingly useless during this time. Billions eventually died as a result of the wars, renegade psykers, daemonic possessions and starvation that ran rampant during this dark period.

Mars eventually found itself cut off from Terra and all the other human-settled worlds during this period, and its leaders could no longer acquire sufficient food or resources to accommodate its large population. Mars was soon consumed by strife as its society drifted into anarchy and its civilisation collapsed amidst the chaos of Old Night.

A new faith began to spread amongst the survivors of the Martian collapse, a religion focused on the technology needed to ensure their continued survival on a world whose carefully constructed biosphere was unravelling before their very eyes. This was the Cult Mechanicus, dedicated to the animistic worship of the Machine God. Under the direction of the Cult Mechanicus' rigid hierarchy of Tech-priests, the cultists set about restoring order to their world.

When the Tech-priests of Mars built their first temples to the Machine God and finally restored order to the surface of the Red Planet after long centuries hiding in the Red Planet's subterranean warrens, they also laid the basis for one of the future military arms of the Cult Mechanicus, which were known as the Titan Legions. The monstrous robotic war engines the Mechanicum later built using ancient technology recovered from the ruins of the prior Martian civilisation to withstand the hostile environment of their homeworld were called Titans.

Since that bygone era, the Titan Legions have become the backbone of the armies of the theocratic government known then as the Mechanicum and now as the Adeptus Mechanicus. Once the ancient Mechanicum agreed to join with Terra and forge the Imperium of Man through the signing of the Treaty of Mars, their power came into the service of the Emperor, who was recognised by the doctrines of the Cult Mechanicus as the Omnissiah, the physical avatar of the Machine God in the mortal universe.

Bristling with potent weapons capable of levelling fortresses and obliterating entire armies of enemy troops, the Titans became the ponderous battleships of the galaxy's planetary battlefields. Revered for their technological advancement and connection to the Machine God's divinity, Titans have a sacredness invested in them by virtue of their antiquity and technical complexity. To the Tech-priests of Mars, the Titans represent something more than just fighting machines, and each one is seen as a personal avatar of their beloved Omnissiah.

When the Cult Mechanicus took to the stars, hoping to find remnants of human knowledge lost to history, they settled on worlds throughout the galaxy. Founding new colonies of the Cult Mechanicus, each of these so-called Forge Worlds became a replica of Mars with its temples to the Machine God, enormous spiring hive cities and innumerable factories and manufactorums. Each of these new worlds was protected by its own Titan Legion.

On Terra, once the Warp Storms abated after the birth of Slaanesh, a new age of rebuilding and resurgence began, the Age of the Imperium. Conquering the various techno-barbarians and warlords that ruled Terra, a far-sighted visionary rose from the ashes. His knowledge of past technologies astounded all who spoke to Him. Known only as the Emperor, this great leader of Mankind united the warring people of Terra and prepared to launch His Great Crusade -- the great reconquest of the galaxy led by his genetically-engineered Legiones Astartes -- the Space Marine Legions.

Treaty of Mars

When the Emperor began His Great Crusade to reunite the worlds of Mankind, the first human colony he encountered was the Red Planet, Mars. Heralded by all those who met him as the Omnissiah, the Machine God Incarnate in the mortal realm, some within the Cult Mechanicus were not entirely pleased with this turn of events.

A few of these malcontents incited a short and bloody insurrection against those within the Cult who called for an alliance with the Emperor, but eventually the opposition was crushed and Mars and Terra were finally reunited after millennia of separate development by the Treaty of Mars in the late 30th Millennium. This agreement formally founded the Imperium of Man as an alliance between Terra and the Cult Mechanicus, and granted the latter the autonomy required to maintain their faith even as the Emperor intended to spread his secular and atheistic Imperial Truth across the galaxy.

With the massive human resources of Terra and the colossal technical and industrial power of Mars, the Emperor could begin His mighty enterprise of reconquering the galaxy on a firm foundation. Many long-forgotten human-settled planets were liberated and many more worlds were settled anew. Over the next 200 Terran years the Imperium of Man rapidly expanded across the galaxy.

Forge Worlds throughout the galaxy were finally able to reestablish communications with Mars once the Great Crusade began and many scientific advances and discoveries of lost technologies were spread across the Imperium as a result. The acknowledgement of Mars as the preeminent centre of the Cult Mechanicus and leader of the Forge Worlds was reaffirmed and Mechanicus spacecraft began to travel between the worlds of the Imperium regularly. The considerable might of the Forge Worlds, including their Titan Legions, were soon added to the forces of the Great Crusade itself.

Horus Heresy

The Great Crusade continued to expand outwards, until the Imperium encompassed nearly the entire galaxy by the early years of the 31st Millennium. At that time, a new and unexpected threat emerged to challenge Mankind's dominance over the galaxy. This was the rebellion against the rule of the Emperor that would be known to later ages as the Horus Heresy.

This revolt was instigated and led by the WarmasterHorus, the greatest and most beloved of the Emperor's sons, the Primarchs. The rebellion began with Horus' virus-bombing of those Space Marines whose loyalties to him were suspect on the world of Istvaan III, and soon took hold amongst half of the Space MarineLegions and many of the Titan Legions serving under the command of the Warmaster, until nearly one-third of the entire armed forces of the Imperium had sworn their allegiance to the Traitors.

Schism of Mars

It is not entirely clear how Horus managed to turn such a significant percentage of the armies under his command against the Emperor, but he was known to be a very skilled and persuasive leader who commanded immense personal loyalty amongst his subordinates. But even before the opening stages of his planned insurrection occurred, he knew he would have to secure the support of the Adeptus Mechanicus and their superior technology and weapons if he was to defeat the Emperor and conquer the galaxy. Horus won over the loyalty of many of the Mechanicum's Tech-adepts after promising them the lost secrets of ancient Standard Template Construct (STC) technology that had been recovered from the worlds of the recently subjugated Auretian Technocracy by the Sons of HorusLegion.

The climate on Mars was full of discontent during this tumultuous time. There were tense relations between the various Techno-Magi with sporadic outbreaks of espionage and violence being committed against the various forge cities that represented the primary sociopolitical units of Mars. There were even unconfirmed suspicions that the Titan Legions had already secretly chosen sides in case of a potential conflict.

Regulus, the Mechanicum's representative to Horus' 63rd Expeditionary Fleet who had already thrown in his lot with the Warmaster's cause, was sent to the Red Planet to secure the tentative support of the Fabricator-General of Mars and the overall leader of the Mechanicum, Kelbor-Hal. Regulus convinced the Fabricator-General of Horus' resolve to support increased autonomy for the Mechanicum against the autocratic rule of the Emperor.

As a show of his appreciation for the Fabricator-General's support, Horus provided information to Kelbor-Hal that allowed the Mechanicum to open a repository of forbidden knowledge known as the Vaults of Moravec, which had been sealed for nearly a thousand standard years. The Emperor Himself had decreed that the vaults never be opened, for they contained innumerable artefacts of technology that had been fashioned or corrupted by the malign power of Chaos in ages past. But the deal was struck, and the Fabricator-General accepted Horus' proposal and joined forces with the Warmaster, assisting the Traitors with all of the most advanced technology of Mankind at his disposal.

When this repository was reopened, there was all manner of forbidden arcane knowledge and weaponry that had obviously been tainted by the corrupting influence of Chaos stored within. Soon the corruption spread throughout the forge cities and temples across the Red Planet as scrap code -- Chaos-contaminated digital source code that was infected with an arcane computer virus -- infested the logi-stacks and Cogitator (computer) archives of the Mechanicum, causing literal Chaos to emerge in any Cogitator system that was networked to one of its infected counterparts.

The Fabricator-General and his so-called "Dark Mechanicum" allies used this disruption to marshal the strength of their forces, intent on bringing the rule of Mars firmly under their control. Infected by this vicious scrap code, the Titans of the Legio Agravides and the Legio Fortidus met their end when their reactors went critical and exploded, destroying their fortresses and eliminating these once-proud Titan Legions from the roster of Loyalist forces. In later years, this night would become known in Adeptus Mechanicus legends as the "Death of Innocence."

Later histories would record that the first blow of the Martian civil war was struck against Magos Mattias Kefra, whose forge city in the Sinus Sabaeus region was housed within the Madler Crater. Titans of the Legio Magna marched from the southern Noachis region and within solar minutes had smashed down the gates of Kefra's forge.

Howling engines daubed in red, orange, yellow and black, decorated with flaming horned skull devices, ran amok within the high walls of the crater, crushing everything living beneath them and destroying thousands of Terran years of accumulated wisdom in a fury of fire. Vast libraries burned and weapon shops that served the Imperial Army troops of the Solar Guard were reduced to molten slag as the indiscriminate slaughter continued long into the night, the Legio Magna 's trumpeting warhorns sounding like the atavistic screams of primitive savages.

Amid the Athabasca Valles, the war machines of the Legio Ignatum and the Burning Stars Titan Legion fought in bloody close quarters through the teardrop landforms caused by catastrophic flooding in an earlier, ancient age of the Red Planet. Neither force could gain the advantage, nor could either claim victory, so after a night’s undignified scrapping, both withdrew to lick their wounds.

Along the borders of the Lunae Palus and Arcadia regions, what previously had been simply a heated debate between the partisans of the Emperor and Horus erupted into outright civil war as Princeps Ulriche of the Death Stalkers unleashed his war engines upon the fortress of Maxen Vledig's Legio Honorum. Caught by surprise, the Legio Honorum lost nineteen Titans in the first hour of battle, before withdrawing into the frozen wastes of the Mare Boreum and seeking refuge in the dune fields of Olympia Undae.

Their calls for reinforcement went unanswered, for all of Mars was tearing itself apart as the plague of war spread across the planet in a raging firestorm, a conflict known as the Schism of Mars by later generations.

The Fabricator-General's betrayal had only begun to unfold, and would soon see the Dark Mechanicum and the Traitor Titans of Mars joining Horus in open war against the Emperor on Terra itself.

The Siege of Terra

The Siege of Terra by the Traitor forces of Horus began with an orbital bombardment by the Warmaster's fleet as the prelude to invasion. After days of shelling, the Astartes of the Traitor Legions landed on the surface of Terra in Drop Pods and advanced on the two spaceports nearest the location of the Imperial Palace to secure them in preparation for the main landings of the Traitor forces. Elements from five of the Traitor Legions participated in the battle, aided by Traitor forces already on the surface.

Despite the brave efforts of the Loyalists, the Eternity Wall and the Lion's Gate Spaceports fell within solar hours to the Forces of Chaos. With them secured, Horus' remaining troops in the Traitor Legions and their Traitor Imperial Army and Dark Mechanicus support forces landed en masse, and the hulking transports carried thousands of troops each. They also brought to the battlefield the terrible Traitor Titans that served the Warmaster's cause and had been infected with the daemonic spirits of Chaos.

The transports' immense size made them prime targets for Terra's Defence Lasers. Although many of the Traitor landing craft were destroyed in-atmosphere, notably the transport vessel carrying the Legio Damnatus, many more made it to the surface, disgorging yet more soldiers, main battle tanks and Traitor Titans to add to the besiegers' strength. They met stiff resistance from the Loyalists as the Imperial defenders knew that the survival of their homeworld, their Emperor, and the entirety of the human race rested on their shoulders.

The siege of the Imperial Palace then began in earnest. Three times the Forces of Chaos scaled the walls, and three times were hurled back by the defenders. Frustrated at this lack of progress, Horus granted the Legio Mortis the singular honour of breaching the walls of the Imperial Palace, amongst whose defenders were the Loyalist Titans of the Collegia Titanica and their hated rivals -- the Legio Ignatum.

Using the many powerful weapons at their disposal, they eagerly set about the task. By virtue of their insane fury they accomplished this near-suicidal endeavour, despite suffering the losses of over thirty Titans in one evening of fierce fighting. The Chaos Warlord-class Titans broke the outer walls and let inwards a flood of Traitors.

But ultimately, the Traitors' assault failed as the final events of the Heresy played out aboard Horus' own flagship, the Vengeful Spirit. The Emperor triumphed during the confrontation between himself and the Chaos-corrupted Warmaster, but only at the cost of his own mortal wounding.

The majority of the Traitor Legions scattered following this disastrous defeat, and the Imperial forces gave chase, unleashing the period known as the Great Scouring. Hunted and pursued, system by system, the Chaos Space Marine Traitor Legions and the Traitor Titan Legions eventually were driven into the Eye of Terror and the worlds they had occupied were reconquered by the Imperium.

Post-Heresy

Warlord-class Titans lead a ground assault

Following the Horus Heresy, the Collegia Titanica continued to serve the reformed Adeptus Mechanicus and the Imperium, bringing its might to bear where it was deemed required. Throughout the Heresy, past rivalries between certain Titan Legions were aggravated into outright hatred once they found themselves on opposing sides.

For millennia they continued to battle the Traitors at every opportunity, and one of their most significant assignments is bearing vigil over the Eye of Terror and guarding Imperial space from Chaos raids and Black Crusades. The Collegia Titanica also protects the Imperium from xenos incursions and other threats.

Although as a result of their political autonomy as part of the Mechanicus they are generally not concerned with any internal quarrels that may occur between the Imperium's sometimes competing Adepta, the Titan Legions do at times interfere at the behest of their Mechanicus masters, whatever their motives may be.

As for the Traitor Titan Legions, following the failure to take the Imperial Palace on Terra, they retreated into the Eye of Terror along with the majority of the Forces of Chaos. From there, as well as from other vantage points like the smaller Warp rift known as the Maelstrom, they continue to wage the Long War against the Emperor, taking part in raids on outlying Imperial worlds, supporting Black Crusades which strike out into Imperial-held space, as well as engaging in battles between the various Chaos factions themselves.

Though somewhat similar to their Imperial counterparts, the Traitor Titan Legions' long exposure to the warping influence of Chaos has changed many of them. In some cases, the crew has merged with their machine, making it impossible to tell where one ends and the other begins. Other crews have given themselves and their weapons over solely to the service of one of the four great Ruinous Powers of Chaos.

The Titan's Machine Spirit (artificial intelligence) and its damned crew become possessed by daemons and are doomed to serve the will of the Chaos Gods in battle for all eternity. Yet Chaos can corrupt Titan Legions at any time, and the ranks of the Traitor Titan Legions have grown over the millennia since the Heresy.

There are two known Titan Legions that have, much to the Collegia Titanica's eternal shame, repudiated their oaths of fealty to the Emperor and willingly betrayed Mankind in recent times -- the Legio Lacrymea and the Adamant Fury Legion.